While single crystal .alpha.-SiC exhibits some degree of optical transmission, polycrystalline .beta.-SiC is substantially opaque. It would be desirable to provide .beta.-SiC which optically transmits, particularly in the visible and infrared range. Optically transmitting SiC would have applications for lenses and domes, light detection and ranging (LIDAR) mirrors, solar collectors and concentrators, cathode covers (in the reactive ion etching systems), and astronomical mirrors. Single crystal .alpha.-SiC is unsuitable for such bulk applications due to its size limitation; thus, it would be desirable to produce optically transmitting .beta.-SiC which can be produced in bulk.
Known bulk silicon carbide is almost invariably opaque, absorbing and scattering light in the visible and infrared regions of the spectrum. Methods of producing bulk SiC include sintering and hot pressing, slip casting and reaction bonding. These methods are unlikely candidates for producing optically transmitting SiC as they suffer from one or more of the following drawbacks: 1) they produce a multi-phase material, e.g., include a Si phase, 2) the material is not theoretically dense, and/or 3) the material is not of high purity and may contain metallic impurities that degrade optical transmission.
A better candidate for producing optically transmitting SiC, either free-standing bulk SiC or thin films on a substrate surface, is SiC produced by chemical vapor deposition (CVD). However, most of the SiC previously produced by CVD is opaque, exhibiting considerable scattering and absorption in the visible and infrared. Applicants are aware of efforts by the U.S. Air Force and General Electric to improve the infrared transmission of SiC by improving process parameters, such as substrate temperature, flow rates, furnace pressure etc. These efforts met with limited success. Although SiC transmitted in the 2-5 .mu.m (infrared spectrum range), the material exhibited considerable scattering which made it unsuitable for use in imaging applications.
It is a general object of the present invention to produce .beta.-SiC in bulk or thin film form having improved optical transmission. Other objects include providing CVD-SiC having heretofore unachieved purity, and electrical resistivity.